The invention concerns an apparatus for detecting the phase and amplitude of electromagnetic waves, more specifically preferably in the optical and in the near infrared and ultraviolet range, comprising at least two modulation photogates which are sensitive to the electromagnetic waves (or photosensitive) and accumulation gates which are associated with the modulation photogates and which are not photosensitive or shaded, and electrical connections for the modulation photogates and the accumulation gates so that the latter can be connected to a reading-out device and the former can be connected to a modulating device, wherein the modulating device increases or reduces the potential of the modulation photogates relative to each other and also relative to the preferably constant potential of the accumulation gates corresponding to a desired modulation function.
Such an apparatus is known by the term xe2x80x98photomixing detectorxe2x80x99 (abbreviated as PMD) from German patent applications Nos 196 35 932.5 and 197 04 496.4 and international patent application PCT/DE97/01956 based on the two applications referred to above.
The above-indicated applications are to the same inventor and were filed for the applicant of the present application, and reference is made to the entire disclosure of those previous applications insofar as described therein are the basic mode of operation, performance and possible uses of photomixing detectors. The present invention therefore does not discuss those fundamental functions of photomixing detectors but is concerned primarily with specific configurations and uses of photomixing detectors, by which the elements that are already known are optimised.
By virtue of the inherent mixing procedure which is implemented upon reception of the light which is modulated, reflected or emitted by an object, by the modulation photogates which are modulated with the same modulation function, the known PMDs are in a position directly to detect the transit time of the electromagnetic waves reflected by the object and therewith, besides lateral locational resolution which is ensured by means of a suitable optical system as in conventional cameras, simultaneously also to obtain items of spacing Information about the recorded pixels. Those PMDs therefore permit direct three-dimensional surveying and measurement of surfaces without the need for expensive evaluation procedures and recording procedures at various angles.
In order to achieve adequate sensitivity and depth resolution in the case of the known PMDs, the pixel surfaces must be sufficiently large so that, during the recording duration of an individual image, sufficient electromagnetic radiation is received from the various surface regions of the object and a suitable number of charge carriers is produced in the photosensitive material as ultimately the spacing information is obtained by way of the different number of charge carriers which occur at different moments in time at the modulation photogates and are accumulated by way of the immediately adjoining accumulation gates.
That entails a certain minimum size in regard to the area of the individual pixels. Problems can also occur with the conventional PMDs by virtue of the fact that very sharp light-dark boundaries of the object are produced in the image. If such a light-dark boundary falls by chance on the boundary region between adjacent modulation photogates, then the different number of charge carriers at the adjacent accumulation gates thus fakes a correlation result which leads to incorrect interpretation in the sense of depth information.
In addition the transit times in photosensitive pixel elements of that kind, which are of relatively large area, are comparatively long so that the band width or the limit value of the modulation frequency is usually only in the region of a few megahertz to a maximum of 100 MHz. Band widths of at least 1 GHz are desired in particular for the use of corresponding photosensitive detectors, for example in the opto-electronic art and in optical signal transmission.
In addition a higher level of functionality and flexible use of the PMD-pixels and PMD-arrays is desirable for different uses, for example for implementing different modes of operation with the same pixels, in particular for reasons of economy.
Having regard to the foregoing, the object of the present invention is to provide an apparatus for detecting the phase and amplitude of electromagnetic waves, having the features set forth in the opening part of this specification, which has a markedly improved band width, in which moreover misinterpretations of light-dark boundaries on imaged surfaces are less probable or even excluded, and with which a higher level of functionality and economy in practical uses is achieved.
That object is attained in that the modulation photogates like also the accumulation gates are provided in the form of long narrow parallel strips in mutually juxtaposed relationship, which group-wise form a PMD-pixel, and wherein the accumulation gates are in the form of reading-out diodes.
The fact that the modulation photogates and the accumulation gates are in the form of narrow long strips and the arrangement thereof in parallel directly mutually juxtaposed relationship results in very short channel lengths for the gates (the modulation gate strip width is referred to as the gate length, from MOS-transistor technology). The free charge carriers produced in or under the modulation photogates drift only transversely with respect to the strip direction by the short distance of the gate length to the adjoining accumulation gate, in which respect that drift is supported by a suitable electrical field on the part of the modulation voltage at the modulation photogates. As a result the drift times fall for example below 1 nanosecond so that accordingly it is possible to achieve a usable modulation band width of 1 GHz. Even if the individual strips of the modulation photogates and also the accumulation gates are relatively narrow, nonetheless by virtue of their corresponding length they can afford a sufficiently large photosensitive area while in addition it will be appreciated that a plurality of alternately arranged, strip-shaped modulation photogates and accumulation gates can be connected together to form a unit with almost a doubling of the optical filling factor. In that way virtually any pixel shapes and pixel sizes can be embodied by strip structures of that kind, without a limitation in terms of modulation band width.
The preferred embodiment of the invention provides that the individual modulation photogates are of a width which is greater than that of the respectively adjoining accumulation gates, while in addition the width of the modulation photogates should if possible be smaller than the diffraction limit of the imaging optical system for the modulated light detected by those elements, and should preferably be of the order of magnitude of the wavelength or a few wavelengths of that light or that electromagnetic wave. This means that, by virtue of diffraction effects, sharp light-dark boundaries can no longer accidentally fall on the region between two adjacent modulation photogates which are modulated in push-pull relationship. On the contrary the small dimensions of the modulation photogates in the transverse direction provide that a shadow or light-dark boundary must be spread over the full width of those gates so that both adjacent gates are equally still acted upon by light. In addition, with very long, correspondingly narrow modulation photogates, it is in any case highly improbable that a light-dark boundary extends precisely parallel to the direction of those strips. With the slightest inclination relative to the strips however at any event the two adjacent modulation photogates which are modulated in push-pull relationship are substantially equally acted upon with light from the light part and the dark part of the respective object whose image is produced.
The strip length of the modulation photogates and also of the accumulation gates should if possible be at least ten times to one hundred times the width thereof. The width of the pixels formed from a plurality of modulation photogates and accumulation gates should overall be approximately of the same order of magnitude as the length, which means that about 10-100 strips are to be arranged in mutually juxtaposed relationship, of which about a third are accumulation gates and about two thirds are modulation photogates. In another embodiment of the invention however it is possible also to arrange three or even more modulation photogates in the form of corresponding strips to improve the potential configuration between each two accumulation gates, in which case the central modulation photogate should be unmodulated. In addition, a preferred embodiment of the invention is one in which, as viewed in the transverse direction with respect to the strips, in a pixel, there are always two modulation photogate strips alternating with an accumulation gate strip, wherein the two immediately mutually juxtaposed modulation photogates are connected in such a way that their potential can be modulated in push-pull relationship with respect to each other, wherein the accumulation gates each have a preferably constant, lower energy potential, that is to say for example positive potential for the photoelectrons, which provides that the charge carriers produced under the two modulation photogates drift predominantly to the side of that modulation photogate which assumes the low energy potential value and from there go to the accumulation gate arranged on that side of the two strips. In that case the two modulation photogate strips which are arranged on respective sides of an accumulation gate are modulated in push-pull relationship, that is to say at a given moment in time an accumulation gate simultaneously receives charge carriers from both modulation photogate strips adjacent thereto while the respectively adjacent accumulation gate is adjacent to two modulation photogate strips which at that moment in time are precisely at a higher potential so that only very few charge carriers go to that accumulation gate. Accordingly each second respective accumulation gate is also connected to one and the same reading-out line and the remaining accumulation gates are connected to another reading-out line, wherein the sum signal of those two lines reproduces the amplitude of the received light while the difference signal directly specifies the value of the correlation signal which arises out of modulation of the received light and simultaneous modulation with the same modulation function of the directly adjacent modulation photogates in push-pull relationship. That also occurs in a completely analogous fashion when using a third modulation photogate which would additionally be arranged between the two above-mentioned modulation photogates and which for example could be disposed at a constant mean potential while the two adjacent photogates could be increased and reduced with the modulation voltage in push-pull relationship relative to the central gate. In that way the potential configuration can also be smoothed somewhat and the level of efficiency in regard to unilateral displacement of the charge carriers could be increased in accordance with the respective current value of the modulation voltage.
In accordance with the invention the accumulation gates are in the form of reading-out diodes.
In the possible voltage reading-out mode the photocharges which are distributed in accordance with the push-pull modulation voltages are stored on the capacitances of the accumulation gatesxe2x80x94in this case on the barrier layer capacitances of the reading-out diodes which are connected in the blocking direction (for example pn-diodes or Schottky diodes)xe2x80x94and ascertained with a high-ohmic reading-out apparatus.
In the current reading-out mode which is preferred herein the arriving photocharge is transmitted directly to the reading-out circuit with a practically unaltered potential of the reading-out electrode.
As in the case of the semiconductor materials which are current at the present time for the uses of this invention, electron mobility is greater than that of the holes or defect electrons, preferably photoelectrons are directionally modulated by the modulation photogates and distributed in accordance with the modulation voltage to the accumulation gates or reading-out diodes. In this case the anodes of the reading-out diodes are preferably at common ground potential while the cathodes are at positive potential and are connected as reading-out electrodes K+and Kxe2x88x92respectively to the reading-out circuit.
The above-discussed two kinds of accumulation gates K+and Kxe2x88x92alternate with each other in a group of accumulator gates and modulation gates which in accordance with the invention operate in parallel and which form a novel PMD-pixel of highest band width, in accordance with the invention, in a manner such that for example a positive modulation photogate voltage results in a photocharge enrichment at the K+-accumulation gates or a photocharge depletion at the Kxe2x88x92-accumulation gates, wherein the accumulation gates are used double by virtue of the double-sided charge accumulation and almost double the optical degree of filling internal to the pixels and noticeably reduce parasitic capacitances.
In a preferred embodiment of the invention two pixels respectively comprising a plurality of parallel strips of modulation photogates and accumulation gates are arranged in directly mutually juxtaposed relationship, here referred to as 2-quadrature pixels. In this respect it is to be pointed out that in the transverse direction the pixels are respectively terminated by an individual modulation photogate adjoining the last accumulation gate strip in that direction, and not by a pair, as is the case between the accumulation gates. If two such pixels are arranged in directly mutually juxtaposed relationship, then those two terminal strips which each form a respective modulation photogate of a respective one of the two pixels come to lie in mutually juxtaposed relationship and the two pixels which are separated in themselves can now be modulated in such a way that the two mutually juxtaposed modulation photogates are modulated in push-pull relationship with respect to each other, which effectively amounts to having the pixel surface area doubled at the same modulation voltage and phase, with a unitary larger pixel being formed from the two individual pixels. As however in this configuration the two halves of that larger pixel can in principle be modulated independently of each other, the modulation function can equally well also be displaced in respect of phase or transit time in regard to the one pixel through 90xc2x0 or a suitable delay TD, in relation to the other pixel. That means that in-phase and quadrature signals are measured at the same time so that in that fashion complete information about the phase position of the correlation function is obtained in parallel and simultaneous relationship.
In that case the accumulation gate connections or terminals are desirably respectively provided at the end of one of the pixels. The modulation photogate connections or terminals are preferably provided in the form of push-pull strip lines from both ends or strip ends of the pixel surfaces, more specifically in particular in the case of especially long strips so in addition by virtue of transversely extending push-pull strip lengths in each case as a multiple and at equal spacings. That prevents the modulation signal being attenuated and deformed over the length of the gate by virtue of the electrical surface resistance of the modulation photogate, so that the ascertained correlation function is also correspondingly deformed.
A particularly preferred embodiment of the invention is one in which four pixels are arranged in a rectangle or square and form a unit, more specifically in such a way that the strips of pixels arranged in the square or rectangle in respective diagonal relationship with each other extend parallel to each other while the strips of the directly adjacently disposed pixels extend perpendicularly to each other, thereby substantially avoiding in particular troublesome mutual overcoupling effects. This embodiment is referred to herein as 4 quadrant (4Q)-PMD-pixels. If the pixels themselves are square then the pixel element composed of the four quadrants is also square and by phase shift of modulation as between diagonally mutually opposite pixels it is possible simultaneously to detect the push-pull correlation values of the in-phase and quadrature signals.
In addition a further embodiment of the invention is preferred in which arranged over the modulation photogates and the accumulation gates are corresponding strip-shaped lenses, specifically therefore cylindrical lenses, which focus the light impinging on the lenses on to the modulation photogates so that the surface components which are occupied by the non-photosensitive accumulation gates also still effectively contribute to the light yield. If the coupled-in modulated light is relatively narrow-band in nature, the strip structure can be so dimensioned for a mean wavelength of such light that the coupling-in factor, in accordance with the wave theory, is markedly greater, or the reflection factor is markedly smaller, than corresponds to the reflection factors in accordance with geometrical optics. In that case it is possible to involve an improvement in the modulation photogates and to promote such a measure.
A plurality of pixels can either be connected together to form a linear array or a matrix array, in which respect a preferred embodiment is one in which arranged over the individual pixels are microlenses which direct the incident light which in part is also directed on to regions which are between the pixels and do not contribute to evaluation through the microlenses on to the photosensitive pixel surfaces.
A 4Q-PMD-pixel, with four times the same or with differing modulation, can measure the points of concentration of the four square sub-pixels and at the same time ascertain the overall phase or transit time of the 4Q-PMD-pixel by averaging of all 4 correlation values. The four individual concentration point transit times supply in that case the gradient or normal vector of the imaged surface element and permit improved interpolation of the 3D-surface to be measured, between the adjacent pixels of an array.
Finally a particularly preferred embodiment of the invention is one in which the pixels, that is to say the individual modulation photogates and the accumulation gates are implemented using CMOS-technology. That is a very inexpensive and well-established technology which permits mass production of corresponding elements and at the same time also allows on-chip and multi-chip module integration of the peripheral electronics such as the evaluation electronics and the modulation electronics.
In CMOS-technology both conventional CMOS-pixels with 2D-functionality (so-called 2D-pixels) and also PMD-pixels with 3D-functionality (so-called 3D-pixels) can be integrated in a linear array or in a matrix array in a mixed configuration. In this case the various, in particular adjacent items of pixel information can be evaluated in a downstream-disposed, data-fusioning and interpolating apparatus, in terms of rapid reconstruction of the complete 3D-color/depth image by means of the items of color information of the 2D-pixels and the 3D-depth and 2D-gray value information of the 3D-PMD-pixels, which affords totally new options in regard to optical measurement procedures and in automation, object identification, security technology and multi-media technology.